The present invention relates to a sheet feed mechanism utilizing a frictional force which is applied to an automatic mail piece processing apparatus, e.g., an automatic mail piece arranging/stamping machine that, for example, detects a postage stamp of mail piece, stamps the mail piece, and thereafter arranges the mail piece such that the surfaces of the mail piece where the postage stamps are stuck are aligned.
Conventionally, sheet feed mechanisms of this type are used widely as the ADF mechanisms of office automation equipments represented by a copying machine and, e.g., one disclosed in Japanese Patent Laid-Open No. 61-106354 is known. In this sheet feed mechanism, as shown in FIGS. 7A, 7B, and 7C, an electromagnetic clutch 54 is provided between a driving shaft 52 of a first feed roller 51 and a gear 53 driven by a motor (not shown). A second feed roller 55 is driven by the driving shaft 52 through gear trains 56a, 56b, and 56c to rotate in the same direction as that of the first feed roller 51 in synchronism with it. The second feed roller 55 is pivotal about the driving shaft 52 as the center. A one-way clutch 57 for allowing rotation of the driving shaft 52 only in the feed direction is mounted, in one of bearings that support the driving shaft 52. A pick-up roller 58 is pressed against by a pinch roller 59 and is normally rotated in a transport direction. A driving shaft 62 of a torque limiter 61 that applies a torque to a reverse roller 60 is driven by the rotation of the pick-up roller 58 through gears 63a, 63b, 63c, and 63d. The driving shaft 62 of the reverse roller 60 is pivotal about a shaft 64 as the center.
When only one sheet 65 is fed to a portion between the first feed roller 51 and the reverse roller 60, since a torque generated by the friction of the sheet 65 with the reverse roller 60 and applied to the reverse roller 60 is set to be larger than the torque of the torque limiter 61, the reverse roller 60 is rotated in the sheet feed direction without causing a slip with the sheet 65. When two or more sheets 65 are transported to a portion between the second feed roller 55 and the reverse roller 60, the torque applied by the torque limiter 61 is larger than the torque generated by the friction between the sheets. Thus, the second and subsequent sheets are pushed back to the feed table, and the sheet 65 which is in contact with the first feed roller 51 is transported. This prevents double transport of the sheets 65.
After the sheets 65 are separated apart, when a sheet position detection sensor 66 detects that the leading end of one sheet 65 is caught by the pick-up roller 58 and the pinch roller 59, the electromagnetic clutch 54 is turned off, and this sheet 65 is transported by the pick-up roller 58 and the pinch roller 59. When the sensor 66 detects the trailing end of the sheet 65, the electromagnetic clutch 54 is turned on after a predetermined period of time, and feeding of the following sheet is started.
The conventional friction type sheet feed mechanism described above poses no problem when the types (thickness, weight, paper quality, and the like) of the sheets to be dealt with are limited and a high processing speed is not required, as in an office automation equipment represented by a copying machine. However, as in an automatic mail piece processing apparatus, when the types of sheets (mail piece) to be dealt with vary and the mail piece must be processed at a high speed (e.g., with a transport speed=3 m/s or more and a processing speed=at least about 10 items/second), following problems arise.
(1) When the sheet position detection sensor 66 is located at only a position on a line connecting the pick-up roller 58 and the pinch roller 59, an error in feed interval is increased due to variations in wait position of the leading end of mail piece which is fed as second or subsequent mail piece. When the processing ability of the downstream unit is considered, the feeding unit must feed the mail piece with at least a minimum interval with which the downstream unit can process the mail piece. Then, a waste interval occurs between the mail pieces due to the error in feed interval, and an improvement in processing speed cannot be achieved.
(2) Since the torque limiter 61 is directly coupled to a shaft coaxial with the rotating shaft of the reverse roller 60, the moment of inertia with respect to the pivot shaft of the reverse roller 60 is increased. When thick (about 6 mm) mail pieces enter, the reverse roller 60 pivots largely, so that the two mail pieces cannot be separated sufficiently. When the torque limiter 61 is directly coupled to the rotating shaft of the reverse roller 60, the moment of inertia with respect to the rotating shaft of the reverse roller 60 is increased. When two or more mail pieces enter, the rotation of the reverse roller 60 is switched from the feed direction to the counter feed direction, and hence it takes time to separate the mail pieces, so that double feed is increased undesirably.
(3) At the start of feeding, when mail piece is to be fed only with the second feed roller 55, if the mail piece is heavy (e.g., about 50 g or more), a slip occurs between the second feed roller 55 and the mail piece, thus easily causing jam. Assume that a third feed roller (not shown) is simply added on the upstream side of the second feed roller 55 in the feed direction and this third feed roller is started/stopped in the same manner as the second feed roller 55. If a short mail piece is fed, when its trailing end is separated from the third feed roller, its leading end does not reach the pick-up roller yet. Then, the third feed roller erroneously transports the second mail piece, resulting in double feed.